Ischemic damage to the central nervous system (CNS) may result from either global or focal ischemic conditions. Global ischemia occurs under conditions in which blood flow to the entire brain ceases for a period of time, such as may result from cardiac arrest. Focal ischemia occurs under conditions in which a portion of the brain is deprived of its normal blood supply, such as may result from thromboembolytic occlusion of a cerebral vessel, traumatic head injury, edema, or brain tumors.
Both global and focal ischemic conditions have the potential for producing widespread neuronal damage, even if the global ischemic condition is transient or the focal condition affects a very limited area.
Although these conditions appear to have similar underlying biochemical sequelae, the time scale over which they produce their respective damage may vary. Thus, in global ischemia, in which cessation of blood flow is transient, though some permanent neuronal injury may occur in the initial minutes following cessation of blood flow to the brain, much of the damage appears several days following the ischemic event. Moreover, certain regions of the brain are selectively vulnerable to the effects of global ischemia (Kirino, Pulsinelli (1982)). Secondary consequences of reperfusion of the tissue, such as the release of vasoactive products by damaged endothelium, and the release of cytotoxic products (free radicals, leukotrienes, etc.) by damaged tissues have been hypothesized to underlie the observed delay in neuronal damage.
Focal ischemia, on the other hand, may be of limited or prolonged duration. In the case of prolonged focal ischemia, as caused by lodgement of a thromboembolus in a cerebral blood vessel, reduction of blood flow to a defined, focal region may be followed by reperfusion to part of the ischemic region, via collateral circulatory pathways. Ischemic cell death following focal ischemia has been reported to be complete 24 hours after the primary ischemic event (Nedergaard, 1987).
Several drug strategies have been proposed for treatment of stroke and other neuronal conditions related to ischemia, and these have been reviewed in recent articles (e.g., Greenberg, Wauquier). Anti-coagulants, such as heparin, have been examined, but with mixed results. Similarly, antivasoconstriction agents, such as flunarizine, excitatory neurotransmitter antagonists, such as MK-801 and AP7, and anti-edemic compounds have shown mixed results, with no clear benefits to outweigh a variety of side effects, including neurotoxicity or increased susceptibility to infection.
Two general classes of vasodilators have been studied for possible treatment of neuronal ischemic damage. Non-specific vasodilators, including papaverine, prostacyclin, pentoxifylline, and nitroprusside failed to demonstrate any clear benefit in reducing ischemic damage. A second general class of vasodilators includes a variety of calcium-antagonist vasodilator drugs. Verapamil and related compounds which prevent calcium entry into smooth and striated muscle appear to be effective only at high drug concentrations, where serious cardiotoxicity effects may ensue. Dihydropyridines, such as nimodipine, have produced mixed results--some neurological improvement may be seen, but increased cerebral edema has also been observed. Benzothiazepines, as exemplified by diltiazem, have shown moderate protective effects, but these drugs also appear to cause undesired side effects.
In general, the drugs mentioned above have been administered prior to or within a few hours of the period of experimental ischemic insult. In clinical practice, particularly in the treatment of stroke, treatment is generally not feasible until well after the ischemic insult. In those studies in which post-ischemia treatment has been given, the treatment paradigms have generally included treatment commencing before the ischemic event and continuing over an extended period of time, such as continuous administration of nimodipine from one hour before until 24 hours following ischemia (Jacewicz), or repeated doses administered before as well as after the ischemic event (Dirnagl, 1990; Bielenberg, 1990). In one study, the NMDA antagonist MK-801 was administered to Mongolian gerbils 24 hours post-ischemia, and neuroprotection was observed (Gill et al., 1988); however, the effects of this compound have subsequently been shown to be a consequence of postischemic hypothermia rather than a direct action on NMDA receptors in this animal model (Buchan and Pulsinelli, 1990).
In summary, drugs which have been proposed to date for the treatment of stroke and other ischemic-related conditions of the brain are either (i) relatively ineffective, (ii) effective only at dosage levels where undesired side effects are observed, and/or (iii) effective only when administered prior to or shortly after the ischemic insult.
In the parent U.S. Pat. No. 5,051,403 and U.S. Pat. No. 5,189,020, filed Nov. 22, 1989, and Aug. 2, 1990, respectively, the applicants have disclosed that omega-conotoxin peptides and related peptides which exhibit binding and N- or omega-type calcium channel inhibitory properties similar to those of omega-conotoxin peptides are useful in reducing neuronal damage related to ischemic conditions. In the above-referenced applications, both of which are incorporated herein by reference, experiments attesting to the efficacy of these compounds were conducted in accordance with standard experimental paradigms for examining neuroprotection. That is, test compounds were administered at the time of or up to 1 hour following the experimentally induced occlusion which caused the ischemic event. In the current application, the applicants show that reduction of neuronal damage can be enhanced when the N-channel blocking compound is administered between 4-24 hours following ischemia, relative to immediate post-ischemia drug administration.